Vegetable oils that are produced by industrial extraction processes are normally subjected to a number of refining steps in order to render them palatable, to remove extraction solvents and/or to improve storage stability. Notable exceptions are cold pressed oils such as extra virgin olive oil.
The most common types of industrially produced vegetable oils are fruit oils (e.g. palm oil) and seed oils (e.g. sunflower oil).
Crude palm oil produced using known methods in the prior art usually contains substantial amounts of components, such as free fatty acids, phospholipids and malodorous substances. These substances usually need to be removed in order to render the oil palatable and stable while in storage.
Oil seeds like linseeds are usually milled for a long time to produce oil and meal. The current methods employed in producing the crude quality seed oils usually contain appreciable quantities of components that need to be removed. Besides the undesired components that need to be removed, solvent residue also needs to be removed from these seed oils to render the seed oils suitable for consumption.
A sequence of oil refining steps is commonly used to produce a fully refined or refined, bleached and deoderised (RBD) oil. These oil refining steps include (i) removing gums (mainly phospholipids) from the oil; (ii) removing free fatty acids; (iii) removing absorbable compounds (this step is commonly referred to as “bleaching,” but it does more than just decrease the absorbance of the oil); and (iv) removing malodorous compounds contain in the oil.
The main component of vegetable oils, animal fats, marine oils and milk fat are glycerides, i.e. esters formed from glycerol and fatty acids. Triglycerides usually represent the bulk of the glycerides contained in these oils and fats. Mono- and diglycerides are usually present in minor quantities. Glycerophospholipids are usually present in higher quantity and they are usually removed in the degumming step in an oil refining process.
The presence of monoglycerides and diglycerides in commercially available refined oils and fats has several disadvantages. One such disadvantage is that the presence of diglycerides will affect the crystallization and melting behaviour of the triglyceride present in these oils and fats (Siew, W L, “Understanding the Interactions of Diacylglycerols with Oils for Better Product Performance”, Palm Oil Developments, 2001: 36, 6-11).
There are methods known to remove monoglycerides and diglycerides from the refined oils and fats. One such method is to remove them by converting them into triglycerides. U.S. Pat. No. 5,061,498 describes a method of converting partial glycerides (including monoglycerides, diglycerides, glycerophospholipids) within the fats and oils to triglycerides. The method comprises the steps of treating the fats and oils with at least two lipases which are different in fatty acid specificity and/or position specificity, with the lipase having different fatty acid specificity being selected from the group consisting of lipases acting on short chained fatty acids, lipases acting on middle chained fatty acids, lipases acting on all fatty acids and lipases acting on unsaturated fatty acids. The said lipase having different position specificity is selected from the group consisting of lipases having no position specificity and lipases having 1,3-position specificity. The method described in this publication is laborious and cannot be used to produce oils containing triglyceride in an amount of close to 100%.
Consequently, there is a need for a refining process that effectively removes monoglycerides and diglycerides from glyceride oil so as to enable cost-effective production of a refined triglyceride oil of high purity.